1. Field of Invention
The present invention relates generally to the field of radio communications and more specifically to the field of detecting unwanted data in a wireless networking system.
2. Discussion of Background
Wireless data networks must function in environments with high radio noise and interference. In order to achieve maximum range, the receivers must operate at extremely low signal levels. As the signal levels move lower, both the noise and interference signals become large compared to the desired signals (the signal-to-noise ratio or “SNR” decreases).
Most available radio systems communicate data in packet form. A radio receiver detects the start of each packet and receives it. Noise and interference can interfere with the correct reception of the packets. In addition, the interference and statistical fluctuation of the background noise can cause the receiver to incorrectly believe a packet is arriving even when one is not. These “false packets” can interfere with the proper operation of the network.
In particular, false packets can reduce the throughput of networks using CSMA (carrier sense multiple access) schemes, which share the available bandwidth with a listen-before-talk style protocol. In such a network, false packets can have at least two detrimental effects. First, during the time that the wireless station believes a false packet is present, it will interpret the medium as busy and will not initiate the transmission of its own data. This will reduce the data flow from that station and slow the network.
False packets can also affect the reception of valid packets. Most communication systems include a predefined preamble code at the beginning of a packet. During normal operation, the receiver uses the preamble to perform a sequence of tasks including antenna selection, gain setting, frequency offset correction, symbol timing recovery, and channel estimation. These actions must be performed in a specific sequence, and must be completed by the time that the data payload in the packet begins. Once the data portion of the packet begins, these preamble activities stop to avoid interfering with reception of the data.
If a false packet causes a receiver to incorrectly commence preamble analysis, the receiver may be unable to synchronize to a valid packet that arrives later, because its receiver sequence is already engaged. In fact, the receiver may not be able to receive a valid packet until processing the false packet is complete or otherwise terminated.
A further complication is raised by terminating reception solely on the basis of a drop in signal strength. In the radio environment, the receive signal may decrease in magnitude over time due to fading. Drops in signal strength are common even on valid packets. Often, packets with significant drops in signal strength can be successfully received if the packet is not abandoned. Therefore, it would be unacceptable to terminate reception on the basis of a drop in signal strength alone.
Because false packets make the medium appear to be busy, and because they obstruct the reception of valid packets, they can significantly reduce the actual data throughput of typical wireless networks. There is therefore a need in the art for techniques and systems to reduce the amount of time a station spends attempting to receive false packets.